1. Field of the Invention
The invention relates to an optical system that comprises an enlarging lens that creates an image on a transparent microtiter plate which receives an analysis preparation. This optical system has a plurality of cup-shaped wells, and an incident light illumination arranged on the lens-side of the microtiter plate for illuminating the analysis preparation received by the microtiter plate.
2. The Prior Art
Optical systems of the type referred to are used for the evaluation of ELISPOT (Enzyme Linked Immuno Spot) assays. This method has been devised for identifying beta cells secreting individual antibodies. This method can be used for the identification of cytokine secretions of special subgroups of leukocytes or T-lymphocytes from peripheral blood as well as of monocytes and granulocytes. In the ELISPOT method, proteins secreted from the cells are identified with antibodies, and point-shaped stains referred to as spots are produced by means of special coloring methods. These spots are permanent and suitable for evaluation, both visually and by means of image analysis. In laboratory routine, ELISPOT assays are typically performed using microtiter plates (MTP), which are also commercially designated as filter plates or multiscreen plates. The microtiter plates comprise small interlinked cups also referred to as cavities or wells. The bottom of the cups are conventionally formed by a filter membrane to which the point-shaped objects subject to the assay adhere. A standard size MTP comprises 96 wells.
To evaluate an ELISPOT assay it is necessary to count the small spots of up to 1,500 per well which are visible on the bottom of the wells of an MTP, the bottom providing the measurement surface. To this effect, automatically operating evaluating devices are used which image the surface of the bottom of the individual wells with about twenty- to forty-fold magnification. These devices take the images with an electronic camera, and evaluate the digitized image data by means of an electronic computer.
The processes of imaging and evaluating the microtiter plates present the problem that reflection outside the filter membrane produces a phantom image of the filter membrane, so that the spots occurring in the marginal area are detected twice, which introduce errors into the measurement process. To prevent this problem from happening, one possibility would include masking the areas lying outside the well with a circular mask. This would however necessitate an exact positioning of the individual wells of the microtiter plate relative to the optical axis. This is a highly complex and difficult task considering that the microtiter plate manufacturers do not guarantee an absolutely accurate positioning of the filter membranes and the adjustment accuracy of an X-Y table necessary for positioning would have to lie in the range of approximately 20 xcexcm. To solve this problem, one proposal includes detaching the filter membranes from the MTP, adhering them to a film and then optically detecting and evaluating the adhered filter membranes. Separating the filter membranes from the MTP prevents the generation of a phantom image by reflection. However, the disadvantages of this approach are that the processes of detaching and adhering require an additional procedure. This procedure results in having individual membranes shifting out of place as the filter membranes are detached, wherein the film material""s service life is limited. On the other hand, recent image analytical devices provide an easy way to determine the position of the adhered filter membranes and cause an X-Y table to travel to the center of the image, if necessary.
In another known system of the type referred to, the positioning of the magnifying objective initially involves detecting the MTP from a major distance without appreciable magnification and then using this image for measuring and storing the exact position of the individual wells. These stored values can be used to position the magnifying objective above the individual wells automatically. This necessitates a relatively high construction effort for the optical system and the positioning table, and additional memory capacity is needed for storing the position-related data.
A microscope for fluorescence, luminescence and absorption tests of minute amounts of samples is known from DE 197 48 211 A1. These minute amounts are made available in microtiter plates with containers for minute amounts arranged in the form of a matrix. To create an image of the individual samples on an CCD-array provided for their detection, an optical system of the type commonly used, is combined with a mini-lens array that is composed of small lenses. These lenses are arranged with the same matrix dimension as the containers of the microtiter plate for the samples. For the incident light illumination, an illuminating device supplies light to a coupling mirror arranged in the path of the light beam. Using the optical system of the microscope, this light illuminates the spots on the microtiter plate that are imaged on the CCD-array.
An illuminating system for a system microscope with a revolver-type lens is known from EP 0 504 940 A2, wherein the light of a light source is aimed at the object to be viewed on the side of the lens via light-conducting fibers in the form of a ring with respect to the optical axis via a ring-shaped condenser lens. In addition, there is also a reversing mirror and a condenser lens on the side facing away from the lens along the optical axis via a lens system. No provision is made in this case for viewing microtiter plates.
German patent No. DE 3734691C2 discloses an illuminating device for a microscope comprising a multitude of flatly arranged illumination elements, which can be controlled individually and preset in a desired illumination pattern for the object to be assayed. The illumination elements, which are designed as active semiconductor sources, are arranged as through-light illumination behind a diffuser disk and can be ignited in a ring-shaped arrangement.
It is an object of the present invention to provide an optical system of the type initially referred to which enables the measurement surfaces of a transparent MTP to be imaged without the occurrence of reflections interfering with the image evaluation.
Another object of the invention is to form an incident light illumination by a diffuse, ring-shaped light source that is arranged concentrically with the optical axis. Another object is to form a second, substantially dot-shaped light source disposed on the optical axis on the side of the microtiter plate facing away from the lens. This light source supplies the analysis preparation with through-light and with a higher light density higher than the one of the ring-shaped light source.
The illumination according to the invention the MTP area lying outside the measurement surface of the filter membrane is imaged completely white. By contrast, the prepared measurement surface is set off against this light background in a lightly gray color. Reflections and material interferences in the area lying outside the measurement surface needing to be evaluated are no longer visible. This effect can be further enhanced by arranging the approximately point-shaped light source behind a diffuser disk, thus accentuating the contrast between measurement surface and background.
Another object of the invention is that the incident light illumination on the lens side can be achieved also by deflecting light, for example diffuse scatter light to the lens side of the microtiter plate. This deflection may take place with the help of reflecting surfaces or light-conducting elements, which are substantially concentrically arranged with the optical axis.
The optical system of the present invention provides fault-free imaging and evaluation of the measurement surfaces of an MTP""s filter membranes. Even minor inaccuracies in locating the measurement surface centrally to the optical axis will not impair the accuracy of measurement so that the demands placed on the positioning accuracy of the table carrying the MTP and movable in the X and Y direction may be less high. Accordingly, this process enables the table to be manufactured more economically. The distinct contrast of the measurement surfaces from the light background surrounding them, makes it possible to determine the position of the measurement surfaces by means of image analysis and to move them automatically to the image center under appropriate table control. An automatic evaluation of microtiter plates is hence readily possible.